1. Field of the Invention
The present invention relates to a microwave output stabilizing apparatus of a microwave oven for maintaining the anode current of a magnetron within a given range by regulating the current flowing into the primary winding of a high voltage transformer, regardless of variations in the input voltage or the temperature of the magnetron, and a method thereof.
2. Background of the Invention
Utilizing the principle of dielectric heating, an inverter-type microwave oven has recently been developed for the purpose of reducing the size and weight of a microwave oven for heating and cooking foods. Such a conventional inverter-type microwave oven is disclosed by Japanese Utility Model Laid-Open No. 71993 (1990). The invention comprises a current detecting circuit which detects the current flowing into the anode of a magnetron. When the output characteristics of the magnetron are varied by excessive heat, the invention determines the amount of variation based on the amount of anode current detected by the circuit, and controls the amount of anode current so that the output of the magnetron can be consistently maintained.
FIG. 1 illustrates another microwave output stabilizing circuit comprising an over-current protecting circuit which protects the components of the appliance from an excess ire current caused by a short of the magnetron, and a circuit, (designated a soft start circuit hereinafter), which provides for smooth initial activation of the magnetron.
In FIG. 1, a rectifier circuit 1 converts power from a commercial AC power source into a constant DC voltage and an inverter circuit 2 inverts the DC voltage into a high frequency voltage by a switching element TR 1 incorporated therein. A high voltage transformer HVT and a magnetron drive circuit 3 step up the high frequency voltage to a voltage sufficiently high to oscillate the magnetron, which thereby forces the magnetron MGT to oscillate. An inverter control circuit 4 detects the amount of current flowing into the primary winding of the high voltage transformer HVT, monitors whether the detected current value is greater than a given reference value, and controls the inverter circuit 2 in accordance with the monitored results. When the voltage applied to the inverted(-) terminal of a comparator COMP1 is higher than the reference voltage supplied to the non-inverted(+) terminal thereof due to undesirable conditions, such as a variation in the input AC power, the output terminal of the comparator COMP1 will be at a low level.
Accordingly, transistor TR2 and transistor TR3 are consecutively turned on and transistor TR1 is turned off. FIGS. 2A and 2B are graphs showing the relationship between the voltage and current at the anode and cathode electrodes of a magnetron according to variations in the temperature of the magnetron.
In FIGS. 2A and 2B, as the temperature of the magnetron rises, the anode voltage or current increases. Consequently, the total microwave out increases. In the conventional microwave output stabilizing circuit described above, there is a problem that the variation in the microwave output due to the variation the input AC power becomes substantial because the predetermined reference voltage applied to the comparator COMP1 is fixed at a constant value. Moreover, another problem is that, at different times, even when similar foods are heated for the same amount of time, cooking conditions vary according to variations in the microwave output due to variations in the temperature of the magnetron.